Vibrating fiber electrometer

ABSTRACT

Improved operation of a vibrating fiber electrometer having a flexible conductive fiber positioned between two deflecting electrodes is achieved by maintaining the pressure of the atmosphere surrounding the fiber at a value where the ratio of the amplitude of vibration of the fiber at its resonance frequency to the amplitude of vibration at twice the resonance frequency is equal to or greater than 5 and having an alternating voltage of frequency equal to the natural frequency of the fiber applied to the deflecting electrodes. Automatic, continuous operation of the vibrating fiber electrometer is attained by projecting the image of the fiber onto a photosensitive detector which develops a signal corresponding to the phase of vibration of the fiber. A servo system coupled to the fiber and the deflecting electrodes and the detector is responsive to the signal to minimize automatically the vibration of the fiber.

llnited States Patent [191 Spokas Nov. 26, 1974 VIBRATING FIBERELECTROMETER [75] Inventor: John J. Spokas, Lisle, Ill.

[73] Assignee: The United States of America as represented by the UnitedStates Atomic Energy Commission, Washington, DC.

[22] Filed: Aug.' 17, 1973 21 Appl. No.: 389,288

Primary ExaminerAlfred E. Smith Assistant Examiner-Ernest F. Karlsen VAttorney, Agent, 0 rTi7mJohn XT-loran; Arthur A.

Churm; Paul A. Gottlieb 57 ABSTRACT Improved operation of a vibratingfiber electrometer having a flexible conductive fiber positioned betweentwo deflecting electrodes is achieved by maintaining the pressure of theatmosphere surrounding the fiber at a value where the ratio of theamplitude of vibration of the fiber at its resonance frequency to theamplitude of vibration at twice the resonance frequency is equal to orgreater than 5 and having an alternating voltage of frequency equal tothe natural frequency of the fiber applied to the deflecting electrodes.Automatic, continuous operation of the vibrating fiber electrometer isattained by projecting the image of the fiber onto a photosensitivedetector which develops a signal corresponding to the phase of vibrationof the fiber. A servo system coupled to the fiber and the deflectingelectrodes and the detector is responsive to the signal to minimizeautomatically the vibration of the fiber.

3 Claims, 6 Drawing Figures 60 CPS REFERE/YCE VOL 7%65 RECORDER i 0.6.6; VOL mes sou/ere PATENTELEEGVZS I974 SHEET 10F 3 9-6 van/m5 PATENTE.2'UY26 I974 SHEET 3 OF 3 VIBRATING FIBER ELECTROMETER CONTRACTUALORIGIN OF THE INVENTION The invention described herein was made in thecourse of, or under, a contract with the UNITED STATES ATOMIC ENERGYCOMMISSION.

BACKGROUND OF THE INVENTION The vibrating fiber electrometer or Shonkaelectrometer, US. Pat. No. 3,133,248, includes a flexible conductivefiber cantilevered on a fixed input electrode. The tip of the fiber ispositioned centrally in the gap between the ends of two fixed deflectingelectrodes with the electrodes and the fiber coplanar.

In practice, an A-C voltage of generally 60 cps is applied to thedeflecting electrodes so that the potential of each electrode is 180 outof phase with the other electrode but of equal absolute magnitude withthe other electrode at several hundred volts peak amplitude. Thisproduces a relatively intense alternating electric field at the end ofthe fiber generally perpendicular to the fibers axis. Thus, if there isa net charge on the fiber, it will be driven into vibration between thedeflecting electrodes. The presence of a charge on the fiber is therebyindicated by a corresponding vibration of the fiber.

Undesirable charges from spurious sources may appear on the fibercausing unwanted vibrations. One

type of undesirable charge is that induced by the deflecting electrodes.Each deflecting electrode is a radiator of an electric field in which isplaced the conductive fiber. The effect of each deflecting electrode isto induce charges to congregate at the tip of the fiber opposite to thepolarity of the field for that electrode. It is for this reason thatthere are two deflecting electrodes. If each is of equal absolutemagnitude and 180 out of phase, the net induced charge on the fiber willbe zero. Any imbalance in voltage levels between the deflectingelectrodes will produce a net charge on the fiber not equal to zeroandcause spurious vibration. To insure balance the prior art electrometeris provided with an AC balance and a phase control for ensuring equalmagnitude of electrode potential. Note that due to the inherentqualities of electrical devices these controls must be continuallymonitored.

Generally, the electrometer is used as a null detector. In the null modea compensating voltage derived from a potentiometer is appliedsimultaneously and equally to the fixed deflecting electrodes tocompensate for any detected charge on the fiber. Alternatively, thecompensating voltage may be added in series with the signal beingapplied to the input electrode upon which is mounted the fiber. Ineither case, the compensating voltage is adjusted until no vibrationoccurs, with the amount of compensating voltage being related to thecharge. In normal operation, each time asignal is detected, causing achargeinduced vibration of the fiber, the compensating voltage must bemanually adjusted, thereby limiting the electrometer to discretereadings.

It is therefore an object of this invention to eliminate the need forA-C balance and phase controls from the vibrating fiber electrometer.

Another object of this invention is to provide for automatic, continuousoperation of the vibrating fiber electrometer.

SUMMARY OF THE INVENTION A vibrating fiber electrometer includes aflexible conductive fiber positioned between two fixed deflectingelectrodes, with the two deflecting electrodes having an alternatingvoltage applied to each so that each deflecting electrode is out ofphase with the other electrode. Operation of the electrometer withoutthe necessity of balancing out induced charge on the fiber and greatersensitivity is achieved by operation with the atmosphere surrounding thefiber maintained at a pressure of value where the ratio of the amplitudeof vibration of the fiber at its resonance frequency to the amplitude ofvibration at twice the resonance frequency is greater than or equal to 5and with the frequency of the alternating voltage applied to thedeflecting electrodes being equal to the natural frequency of vibrationof the fiber. Automatic and continuous operation of the electrometer isprovided by projecting the image of the fiber onto a photoconductiveelement which develops a signal corresponding to the phase of vibrationof the fiber. Coupled to the photoconductive element and to the fiberand the deflecting electrodes is a servo system which is responsive tothe signal developed by the photoconductive element to minimizeautomatically fiber vibration.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a representation of thespatial relationship of the elements of a vibrating fiber electrometer;

FIG. 2 is a curve of the potentials of the deflecting electrodes in abalanced condition;

FIG. 3 and FIG. 4 are curves of the potentials of the deflectingelectrodes in an unbalanced condition;

FIG. 5 is a set of curves showing the frequency response of arepresentative fiber; and

FIG. 6 is a schematic of the automatic compensating system.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there isshown the spatial relationship of the elements of the vibrating fiberelectrometer. Deflecting electrodes 10 and 11 are coupled to A-C voltagesource 13. Fiber 15 is generally cantilever mounted on and coupled toinput electrode 16 and its tip 17 is positioned between electrodes 10and 11. For

example, in electrometers using the features of this invention, fiber 15was approximately 3 mm. long and 4 microns thick, and was made of quartzcoated with a conductive material such as gold. The fiber wassufficiently flexible so that it could vibrate. The elements of avibrating fiber electrometer are more particularly described in US. Pat.No. 3,133,248.

In operation, A-C voltage source 13 supplies an A-C voltage ofparticular frequency to deflecting electrodes 10 and 11, whichalternately reverses the polarity on deflecting electrodes 10 and 11according to the particular frequency of the applied voltage. Thus, thevoltage level of deflecting electrode 10 ideally will be 180 out ofphase with the voltage level of deflecting electrode 11. In response tothe applied A-C voltage each deflecting electrode produces analternating electric field at the tip 17 of fiber 15. Since fiber 15 isa conductive fiber, the effect on fiber 15 due to the presence of anelectric field generated by deflecting electrode 10 is to induce acharge at the tip 17 of opposite polarity to that of the electric fieldand to then attract fiber 15 to defleeting electrode 10. The same effectresults from the electric field generated by deflecting electrode 1 l.The prior art method of operating the electrometer required the electricfield produced by each deflecting electrode by maintained equal inabsolute magnitude and 180 out of phase, as shown in FIG. 2, toeliminate induced charges on fiber 15 by creating a net zero field attip 17. Curve 20 is a curve of the A-C voltage applied to deflectingelectrode and curve 21 is the A-C voltage applied to deflectingelectrode 11. When the magnitude and the phase of the A-C voltagesapplied to the deflecting electrodes are balanced, the net inducedcharge on fiber due to the electric fields produced by deflectingelectrodes 10 and 11 will be zero.

Referring to FIGS. 3 and 4, there is shown a graphical representation ofconditions of imbalance which may exist between the voltage levels ofdeflecting electrodes 10 and 11. Curves 25 and 27 represent the responseof v deflecting electrode 10 and curves 26 and 28 represent the responseof deflecting electrode 11. In FIG. 3 the imbalance results fromdeflecting electrode 10 having a greater voltage amplitude swing thandeflecting electrode 11. Since the sum of electric fields at tip 17,assuming it is centered between the deflecting electrodes, will not bezero, a charge will be induced at tip 17 opposite in polarity from thatof deflecting electrode 10, and tip 17 will be attracted to deflectingelectrode 10. One-half cycle later when the polarity of deflectingelectrode 10 is reversed, the induced charge at tip 17 will also bereversed and tip 17 will again be attracted to deflecting electrode 10.Thus, the frequency of vibration of fiber 15 due to the induced chargecaused by the amplitude imbalance illustrated in FIG. 3 will be twicethe frequency of the applied A-C voltage to deflecting electrodes 10 and11. To correct this problem, the prior art electrometer included an A-Cbalance control to equalize the absolute amplitudes of deflectingelectrodes 10 and 11. Note also that if fiber 15 is not centered betweendeflecting electrodes 10 and 11 an imbalance will occur which will alsorequire the use of the A-C balance control of the prior art.

In FIG. 4 the imbalance results from slight phase shifts due to theinherent qualities of the electrometer circuitry. The amplitude peaks Aand B of curve 27 occur before the corresponding peaks C and D of curve28 so that a net imbalance will occur. At peak A an induced charge willbe present at tip 17 due to deflecting electrode 10 having a greaterabsolute potential than deflecting electrode 11 and tip 17 will beattracted to deflecting electrode 10. This is repeated one-half cyclelater at peak B. At peak C an induced charge will be present at tip 17due to deflecting electrode 11 having a greater absolute potential thandeflecting electrode 10 and tip 17 will be attracted to deflectingelectrode 11. One-half cycle later, at peak D, this is repeated. Thusthe frequency of vibration of fiber 15 due to the induced charge causedby the imbalance illustrated in FIG. 4 will be twice the applied A-Cfrequency. The prior art electrometer included a phase" control toeliminate this phase imbalance.

Referring to FIG. 5 there is shown the frequency response of a typicalfiber, approximately 4 microns thick and 3 mm. long, having a maximumresponse at natural frequency of vibration of 240 cps. To obtain theresponse, the fiber was positioned between two deflecting electrodes towhich an alternating voltage of varying frequency was applied. Curve 30shows the response with the pressure of the environment surrounding thefiber at one atmosphere and curve 34 shows the response at a pressure of1 mm. of mercury. For each curve the response illustrated is in terms ofamplitude of vibration and not frequency of vibration of the fiber.

Remembering that fiber variations caused by unbalanced applied A-Cvoltage conditions will have a frequency of vibration twice thefrequency of the applied A-C voltage, consider curve 30. Under the priorart practice of operation at 60 cps, it can be seen that the amplitudeof vibrations at 60 cps at point E on curve 30 are much less than theamplitude of vibrations at cps at point F on curve 30. If there is aninduced charge on the fiber caused by an imbalance, vibrationsassociated with the induced charge, which will occur at 120 cps, willovershadow vibrations associated with charges applied at the input whichoccur at 60 cps since the fibers amplitude of response is much greaterat 120 cps. Thus, for an electrometer operating according to prior artpractice, it is necessary that no imbalance in the applied A-C voltagebe permitted, and the A-C balance and phase controls are indispensable.

Consider the frequency response of a typical fiber at 1 mm. of mercuryas shown by curve 34. Reducing the pressure reduces the damping effecton fiber vibration caused by the atmosphere so that distinct amplitudepeaks appear in the response of the fiber. At the resonance peak of 240cps at point G on curve 34 the amplitude of response is significantlygreater than that at any other frequency. Therefore, if the applied A-Cvoltge on deflecting electrodes 10 and 11 is at 240 cps and the pressureis at 1 mm. of mercury, the amplitude of the applied A-C voltage can bereduced as much as tenfold over that needed at one atmosphere ofpressure to achieve the same amplitude of vibration, and thus the samecharge sensitivity of the electrometer. Also,

at an applied A-C voltage of 240 cps, if there happens to be a net nonzero induced on the fiber by an unbalanced applied A-C voltage,vibrations of the fiber will be at twice the frequency of the appliedA-C voltage. This would be 480 cps as shown at point H on curve 34. At480 cps the response of the fiber in terms of amplitude of vibration isseveral orders of magnitude less than that at the natural frequency ofthe fiber at point G on curve 34. Therefore, no detectable motion of thefiber will result due to induced charge on the fiber caused by anunbalanced applied A-C voltage. In an electrometer employing a typicalfiber, and operating with the A-C voltage applied to the deflectingelectrodes at the natural frequency of the fiber and at a pressure ofthe atmosphere surrounding the fiber at 1 mm. of Hg, the need forremoving such imbalance is negated and the A-C balance and phasecontrols of the prior art electrometer become extraneous.

As the pressure is reduced below 1 mm. the fiber response ratio, whichis the ratio of the maximum amplitude of vibration of the fiber at theresonance frequency of the amplitude of vibration of the fiber at twicethe resonance frequency, increases, and as the pressure is increasedabove 1 mm. the fiber response ratio decreases. Elimination of the needfor A-C balance and phase" controls is realized when the fiber responseratio is equal to or greater than 5. At this ratio the amplitude ofvibrations of the fiber at twice the resonance frequency will notmaterially distort the electrometer response.

Referring to FIG. 6 there is shown an apparatus for providing continuousautomatic operation of the vibrating fiber electrometer. Conductivefiber 40 is cantilever mounted on input electrode 41. Fiber 40 iscentrally located between deflecting electrodes 42 and 43 which arecoupled to an A-C voltage source 44. When a charge to be detected isapplied to fiber 40 via input electrode 41, fiber 40 vibrates at thefrequency of an applied alternating electric field resulting from an A-Cvoltage being applied to deflecting electrodes 42 and 43 by AC voltagesource To provide continuous operation of the electrometer it isnecessary for a compensating D-C voltage to be applied either todeflecting electrodes 42 and 43 or fiber 40 to balance out the appliedinput charge and stop the fiber from vibrating. The value and polarityof the compensating voltage'is a measure of the charge on the fiber.

The amplitude of vibration reflects the magnitude of charge on thefiber. However, the phase of vibration depends only on the polarity ofthe charge on the fiber. Consider the condition when the fiber carries apositive charge. If the polarity of deflecting electrode 42 at thatinstant is positive and the polarity of deflecting electrode 43 isnegative, the fiber will experience a force toward electrode 43. Half acycle later when the polarity of deflecting electrodes 42 and 43 isreversed, the fiber will experience a force in the direction ofelectrode 42. If the fiber carries a negative charge, everything repeatsas before except the fiber will be attracted to the deflecting electrodeof opposite polarity than when the fiber carried a positive charge. Thusthe only difference is that the phase of vibration is shifted by 180relative to the vibration resulting from a positive charge. Therefore,the phase of vibration carries the information concerning the polarityof charge existing on the fiber and hence the polarity of compensatingvoltage necessary to null the charge on the fiber. If the existingcharge is positive, the fiber will vibrate as described above, 180 outof phase with vibration associated with a negative charge, and anegative compensating voltage should be applied to the fiber or one ofthe electrodes to null the charge on the fiber.

The disclosed device is a means of detecting this phase as well as theexistence of vibration and using that information to apply automaticallythe proper compensating voltage to eliminate fiber vibration. Usinglight source 45 and lens 46, the image of fiber 40 is projected onto aphotoconductive device 47. Photoconductive device 47 includes twophotoelectric cells 48 and 49 which are otherwise identical. The twocells 48 and 49 are deposited side by side on the same substrate with anapproximately 0.05 inch gap 50 therebetween. This double cell is placedin the focal plane of the objective of lens 46 so that the image offiber at rest is projected onto and coincides with gap 50. When fiber 40is vibrating its image will alternately move across cell 48 and thencell 49 according to the phase of vibration. Cells 48 and 49 have acommon electrode 511. The other electrode of cell 48 is held at positivepotential by battery 52, while the other electrode of cell 49 is held atan equal negative potential by battery 53. In this manner, the sign ofthe output signal at electrode 50 indicates upon which cell the image isencroaching.

Therefore, when the fiber is vibrating, the output signal 6 produced byphotoconductive device 47 will be an altemating signal synchronizedprecisely with the motion of the fiber. The phase of the output thusindicates the sign of the charge on the fiber giving rise to itsvibratron.

The output of photoconductive device 47 is applied to automatic controlsystem 55, where it is amplified by amplifier 56 and then impressed onthe reference windings of servo motor 57 which may be of the type foundin a recording potentiometer. Servo motor 57 drives a sliding contact 58of a slide wire potentiometer 59 and the output of potentiometer 59provides the compensating DC potential for the electrometer, since it iscoupled to A-C voltage source 44 via lead 60 and resistors 61 and 62.The action of servo motor 57 driving sliding contact 58 varies the DCoutput of potentiometer 59. The effect of the DC, generated bypotentiometer 59 and coupled to A-C voltage source 44 and deflectingelectrodes 10 and 11, is to induce a charge on one deflecting electrodewhile the other is held constant to negate the vibration of the chargedfiber. The output of potentiometer 59 is also coupled to a recordingpotentiometer or strip chart recorder 63 which provides a record of thecompensating voltage applied to deflecting electrodes 42 and 43. In thismanner, an automatic, continuous record of compensating voltages ismaintained, thereby indicating continuously and automatically the chargepresent on the fiber.

If, alternatively, the compensating D-C is to be applied to the fiber,then the output of potentiometer 59 would be coupled in series to fiber40. It is apparent balance out the applied charge, there must be anotherconductive surface or electrode upon which charge is stored to form thecompleted circuit. This is analogous to charging a capacitor wherecharge is applied to one plate and, although no current flows directlyfrom plate to plate, a charge will appear at the other plate. The fiberis the other electrode which forms the path to complete the circuit forcharge applied to a deflecting electrode and vice versa. Therefore, itis permissible to describe the coupling of the potentiometer to theelectrometer by saying that it is coupled to and the compensating DC. isapplied to both the deflecting electrodes and the fiber, as each isnecessary for the DC. to be applied to provide a complete circuit forthe balancing of applied charges.

Note that a photoconductive element signal of one phase drives the servomotor in one direction, while a signal of the opposite phase causes thereverse rotation of the motor. Thus, a compensating potential isproduced of the appropriate polarity to offset the signal on the fibercausing it to vibrate. As long as the fiber is vibrating, there will bea net torque out of the servo motor until precisely the correctcompensating voltage is developed by the balancing potentiometer.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of operating a vibrating fiber electrometer having a pair ofelectrodes and a conductive flexible fiber having a charge appliedthereto, said fiber being positioned between said electrodes being freeto vibrate therebetween and having a particular natural mechanicalresonance frequency, including the steps of:

a. maintaining the pressure of the atmosphere surrounding said fiber ata value wherein the ratio of the amplitude of vibration of said fiber atthe natural mechanical resonance frequency to the ampliresponding inphase to the phase of fibration of said fiber, and

c. in response to said second alternating voltage, ap-

plying to said fiber and said electrodes a D-C voltage of valuesufficient to minimize vibrations of said fiber.

3. The method of claim 1 wherein said fiber is made of quartz coatedwith a conductive substance and is approximately 3 mm. long and 4microns thick and the pressure of the atmosphere surrounding said fiberis maintained at a value less than or equal to 1 mm. of

mercury.

1. A method of operating a vibrating fiber electrometer having a pair ofelectrodes and a conductive flexible fiber having a charge appliedthereto, said fiber being positioned between said electrodes being freeto vibrate therebetween and having a particular natural mechanicalresonance frequency, including the steps of: a. maintaining the pressureof the atmosphere surrounding said fiber at a value wherein the ratio ofthe amplitude of vibration of said fiber at the natural mechanicalresonance frequency to the amplitude of vibration of said fiber at twicethe natural mechanical resonance frequency is equal to or greater than5, and b. applying a first alternating voltage to said electrodes withthe frequency of said first alternating voltage being equal to saidnatural mechanical resonance frequency of said fiber.
 2. The method ofclaim 1 further including the steps of: a. projecting the image of saidfiber onto a photosensitive detector, b. with said fiber vibrating,developing a second alternating voltage by said photosensitive detectorcorresponding in phase to the phase of fibration of said fiber, and c.in response to said second alternating voltage, applying to said fiberand said electrodes a D-C voltage of value sufficient to minimizevibrations of said fiber.
 3. The method of claim 1 wherein said fiber ismade of quartz coated with a conductive substance and is approximately 3mm. long and 4 microns thick and the pressure of the atmospheresurrounding said fiber is maintained at a value less than or equal to 1mm. of mercury.